This invention relates to biomedical instrumentation systems typically used in biomedical research and patient care applications, and more specifically to a biomedical apparatus for measuring and evaluating physical variables associated with biologic or biomedical systems such as, for example, pressure, temperature, partial pressure oxygen (PO.sub.2), carbon dioxide (CO.sub.2), humidity, friction, force (weight and mass), displacement (linear and angular), radiant energy (optical) or blood flow. The present invention includes at least one biomedical or physical sensor that measures physical variables, at least one transducer that generates electrical signals representative of the measured physical variables, electronic circuitry for processing and evaluating signals generated by the transducer, and a display assembly for providing information based upon the physical properties exhibited by the biomedical system. The discussion of the preferred embodiment of the present invention sets forth a biomedical apparatus that is adapted to measure pressure at any interface between two surfaces to determine and evaluate physical stresses. For example, the apparatus can be used to measure pressure at an interface between an individual's body and a support surface such as a mattress to determine and evaluate physical stresses exerted on the body. However, it will be appreciated that the present invention can be readily adapted to monitor and evaluate a wide range of physical properties such as those listed above by employing appropriate physical sensors and transducers to measure the desired physical properties.
Pressure sores, also referred to as decubitus ulcers, bed sores and trophic ulcers, are a traumatic condition that often appear on the body of individuals who are disabled or neurologically impaired. One of the primary factors contributing to the formation of pressure sores is tissue deformation which causes occlusion of blood flow. Tissue deformation itself is difficult to measure, but it can be evaluated by examining the principle forces of pressure, shear and friction which act together to distort and deform the tissues. When these forces are unevenly distributed over an area of the body, the tissues deform and distort, which, in turn, may produce severe or prolonged circulatory interference through the collapse of the vascular beds.
When sitting or lying on a surface, the underlying skeletal structure provides an anvil against which the soft tissues can be compressed and deformed. Where the tissues are in a relatively thin state, for example, over the bony prominences of the body, the ability to relieve pressure by dissipation in the tissues is noticeably reduced. Therefore, bony prominences that are sparsely covered by only a thin layer of tissue are highly vulnerable to harmful pressure. Uneven pressure distribution also causes internal shearing effects that are hazardous to living cells. Deformation of tissues caused by shearing forces can occur wherever friction exists between the skin and an external object (e.g., a bed sheet). Shear also can occur when a high pressure area is adjacent to a low pressure area, thereby creating a pressure gradient that causes internal shearing effects.
Pressure sores generally result when shearing forces are exerted on an area of the body hat also is exposed to high pressure. Areas of the body that are particularly susceptible to pressure sores include tissues over the sacrium, ischial tuberosities, greater trochanters, external malleoli and heels. Other sites that are at risk for pressure sores often are based upon a particular patient's position and posture. Increased pressures under certain bony prominences depend upon postural position and changes affecting the tilt, obliquity and rotation of the pelvis when seated. In a lying position, more areas of an individual are prone to develop pressure sores because of the number of bony prominences that may be involved in weight bearing. Pressure of sufficient severity to impair local circulation in an immobilized patient causes local tissue anoxia (ischaemia) that progresses, if unrelieved, to necrosis of the skin and subcutaneous tissues within hours.
In an effort to distribute the external forces exerted on an individual, numerous types of cushions and mattresses have been developed. While no single pressure sore prophylactic device or procedure can prevent sores from ultimately developing, selective use of various procedures and devices minimizes the risk of pressure sore formation. To provide the best proper care for the patient, a clinician must possess knowledge of the complexities of pressure development, carefully observe the patient and compare the performance of different products and regimes.
To assist in this process, quantitative methods of pressure measurement are useful to evaluate and prescribe equipment designed to prevent or minimize pressure sore development. While pressure care regimes and equipment for postural control and pressure relief are abundant, pressure measurement devices and methods for understanding and quantifying the problem are still sparse. Those pressure measurement devices currently available to measure interface pressures employ a variety of different techniques to generate qualitative, semi-quantitative and quantitative results. Examples of such devices include those involving flexible sheets impregnated with acid indicators, electromechanical types using resistive, inductive or capacitive changes, strain gauges, pneumatics and electro-pneumatics. Some of these devices are sensitive to temperature and exhibit hysteresis owing to the nature of the materials used in their construction. These drawbacks limit their use in continuous or repetitive measurements because, for example, changes in temperature in the operating environment can introduce unacceptable drift in the output signal and adversely impact the sensitivity of the device. In practice, it has been difficult to design an ideal transducer for interface pressure measurement, and compromises between one or more of physical factors and safety, cost and ease with which a device can be used have become accepted in the industry.
Commercially available pressure measurement systems often employ a large array of sensors to provide mapping over the complete area of a support surface. The large number of sensors are monitored and evaluated by a computer system (e.g., a desktop or laptop computer system) which provides sufficient processing power to process the large amounts of data generated by the sensors. An example of such a system includes the "Xsensor" pressure mapping system sold by Roho, Inc. of Belleville, Ill. assignee of the present invention. These types of pressure measurement systems are not readily portable, and require considerable setup time and training for proper operation. Furthermore, a matrix (mat) of sensors is potentially the least accurate method of measuring pressure. A large mat of pressure sensors tends to introduce a large artefact into the pressure readings since the mat becomes a support surface in itself as a result of its hammock effect distributing the natural conformity of the support surface. Additionally, calibration of such as system is complex and requires considerable time.
Other pressure measurement techniques involve taping individual sensors directly onto the skin over a bony prominence where the highest pressures occur. Inaccuracies can arise in such an arrangement which influence the results if the sensors alter the pattern of stress so as to create a perturbation effect between the surfaces. Moreover, this procedure is time consuming, and requires advanced training and experience on the part of the clinician to properly position the sensors. As a result, single sensors are not practical for clinical practice.
Many therapists, nurses, doctors and patients cannot afford costly and complex systems such as the mat arrays which also are time consuming to use. However, they require information that helps identify problems, provides feedback about a particular surface and provides data as to whether the limits for tissue tolerance have been exceeded. This information is essential to properly compare support surfaces such as cushions and mattresses, and to fit and adjust seating systems and wheelchairs for a particular patient. Such pressure information also would be useful to inform and educate a caregiver or user about the best methods of pressure relief and their importance.
Therefore, it is desirable to develop a biomedical apparatus and method that improves chances of preventing pressure sores and discomfort, and overcomes the problems of the mat array pressure systems and the single sensor pressure systems. The apparatus preferably is an inexpensive, lightweight, portable device that is sized to fit in a person's hand or pocket. The biomedical apparatus should provide the clinician with a simple and quick indicator of a potential problem so that further investigation can be initiated, if required. It also should assist the clinician in monitoring the performance of support surfaces and comparing different products. The apparatus preferably is easy to use, self-calibrating, and does not require connection to a separate computer.
The biomedical apparatus preferably can be used as an assessment and prescription tool that provides a professional approach to pressure care situations. It should enable comparison and quantification of various support surfaces as part of the clinical assessment process, and facilitate the setup of pressure care equipment such as cushions, mattresses and stump sockets to ensure that optimum pressure relief is achieved. The device also should standardize procedures for assessment and prescription of pressure care devices to ensure continuity of assessment standards. The apparatus also should function as a research tool that allows for collection of historical data that can be compiled for retrospective analyses, as well as an educational and training tool for patients and students. The apparatus preferably provides biofeedback for the patient to reinforce strategies for weight shifts or reductions, and encourages compliance with the correct equipment. This biomedical apparatus also must be cost effective, and help to assist in the assessment of the most appropriate support device is prescribed for the patient. The device also can provide quantitative data for justifying and supporting applications to funding sources.
The hand-held unit preferably includes a removable sensor module that allows for one or more biomedical or physical sensors of various types to be connected to electronic circuitry disposed in the unit via one or more transducers. This configuration allows for the biomedical apparatus to be easily adapted to measure any of a wide range physical properties associated with a biologic or biomedical system, such as, for example, pressure, temperature, partial pressure oxygen (PO.sub.2), carbon dioxide (CO.sub.2), humidity, friction, force (weight and mass), displacement (linear & angular), radiant energy (optical) or blood flow.